JPH07150320A - Hot dip metal coating method and device thereof - Google Patents

Abstract

PURPOSE:To prevent the occurrence of trouble on the outward appearance of surface accompanying the plated surface defect, particularly alloying reaction by preventing the generation of scum, dross, etc., on the plating bath surface at the time of dipping a steel sheet into the plating bath through a snout part. CONSTITUTION:The generation of the scum on the bath surface is prevented by supplying atmospheric gas controlled with the oxygen concn. and the dew point is supplied just above the plating bath or into the plating bath in the snout part 2, through which the steel sheet 1 is dipped into the plating bath 3 in the hot dip metal coating. By this method, particularly, in the alloying treated steel sheet, a galvannealed steel sheet having extremely excellent external appearance of the surface can be obtd.

Description

Detailed Description of the Invention

[0001]

The present invention relates to hot dip galvanizing, hot dip galvanizing, hot dip aluminum coating, hot dip galvanizing-
The present invention relates to a method for hot-dip metal plating such as aluminum alloy plating and an apparatus therefor.

[0002]

2. Description of the Related Art Conventionally, as a hot dip metal plating method, a steel sheet to be plated is heated and annealed, an upper part thereof is connected to an annealing furnace, and a lower end thereof is passed through a snout immersed in the hot dip metal plating bath to form a hot dip metal bath There is a method of obtaining a hot-dip metal-plated steel sheet by immersing it inside, changing the direction of travel by a pot roll in a bath and pulling it upward, and controlling the amount of hot-dip metal plating adhered by a gas drawing method. When the steel sheet is immersed in the plating bath through the snout portion, if foreign matter such as dust, scum, dross, fragments of annealing furnace refractory bricks, powder, etc. adheres to the steel plate surface or plating bath surface, plating surface defects and alloying Problems such as abnormal reaction occur.

In order to solve these problems, various methods have been conventionally proposed. For example, Japanese Patent Publication 2-25
As in Japanese Patent Publication No. 981 and Japanese Patent Publication No. 3-49981,
As described in JP-A-61-186463 and a method of preventing the generation of scum by setting an atmosphere of hydrogen and oxygen in the atmosphere to be introduced into the snout to an inert gas atmosphere having a specific concentration or less, dipping into the snout. Molten zinc is discharged toward the steel plate width direction from the discharge port arranged on one side in the width direction of the steel plate, and floating substances such as floating dross on the bath surface are stripped along the strip width direction in the other side of the snout width direction. To the side of
In addition, a method is known in which floating material is collected from the other end side to prevent steel plate plating defects due to floating dross in the snout.

Further, as in JP-A-4-276051 and JP-A-4-276052, an inert gas is applied on the plating bath surface from one end side to the other end side in the width direction of the steel plate immersed in the snout. By spraying the scum on the surface of the bath and discharging the scum on the other end to the outside of the snout, and the method of preventing the scum from adhering to the steel sheet and Japanese Patent Publication No. 60-2654. Thus, by providing a slit nozzle across the entire width of the steel plate below the surface of the molten metal plating bath and ejecting the molten plating solution toward the steel plate in the vicinity where the steel plate enters the plating bath, the surface of the plated steel plate is It is intended to prevent the adhesion of foreign matter.

However, each of the methods has its respective drawbacks, and cannot prevent foreign matter from adhering to the steel sheet.
That is, Japanese Patent Publication No. 2-25981 and Japanese Patent Publication No. 3-4
In the method according to Japanese Patent Publication No. 9981, the dew point is not controlled, and the scum generation can be reduced but cannot be completely eliminated by simply controlling the hydrogen and oxygen atmosphere gas composition in the snout. If it is possible, there is a problem in that it is impossible to remove foreign matter that adheres to the steel sheet and is brought into the plating bath surface from the inner wall of the snout and the like. Further, JP-A-61-186463,
JP-A-4-276051 and JP-A-4-276052
Although the method disclosed in the publication can obtain the effect of reducing the amount of foreign matter on the bath surface, a part of it adheres to the steel plate in the process of wiping the foreign matter from one end side to the other end side inside the snout, so the foreign matter adhesion is completely prevented. There was a problem that it did not become.

Further, according to the method disclosed in Japanese Patent Publication No. 60-2654, it is necessary to eject the plating bath with a strong pressure in order to remove the foreign matter in the vicinity of the steel sheet. Since the plating droplets from the slit nozzles on both sides collide with each other, the effect of eliminating foreign matter is lost, and problems such as entrainment of foreign matter due to ripples in the plating bath surface occur.

[0007]

SUMMARY OF THE INVENTION The present invention solves the problems described above, and when a steel sheet is immersed in a plating bath through a snout portion, the oxygen concentration is directly above the plating bath in the snout or in the plating bath. And by supplying atmosphere gas with controlled dew point, it is possible to prevent the generation of scum and dross on the surface of the plating bath, thereby preventing problems of plating surface defects, especially surface appearance problems due to alloying reaction. The purpose is.

[0008]

Means for Solving the Problems In order to achieve the above-mentioned objects, the gist of the invention is as follows: (1) A steel sheet is immersed in a plating bath at a snout portion of hot dip metal plating. A molten metal plating method, wherein scum is prevented from being generated on the bath surface by supplying an atmospheric gas whose oxygen concentration and dew point are controlled directly above the plating bath in the snout. (2) The steel plate is immersed in the plating bath at the snout portion of the molten metal plating. By supplying an atmosphere gas whose oxygen concentration and dew point are controlled into the plating bath inside the snout, it is possible to prevent scum generation on the bath surface. Characterized hot-dip plating method. (3) A steel plate is immersed in the plating bath at the snout portion of the molten metal plating. A device for supplying an atmosphere gas whose oxygen concentration and dew point are controlled is provided directly above the plating bath in the snout or in the plating bath. It is in a hot-dip galvanizing machine.

The present invention will be described in detail below with reference to the drawings. FIG. 1 is a vertical sectional view showing a plating process according to the present invention. In the continuous hot-dip galvanizing apparatus, as shown in FIG. 1, a steel plate 1 discharged from a continuous annealing furnace passes through a snout 2 and is immersed in a plating bath 3 in a plating tank, and then a pot roll 4 and then a plating bath 3 Then, the gas wiping nozzle 5 outside the bath wipes off the excessively deposited plating layer on the surface, and the surface of the steel plate 1 is plated with a predetermined thickness. When the alloying treatment is performed thereafter, an alloying heating device is provided.

[0010]

By the way, conventionally, in hot dip galvanizing, a small amount of Al is added to the plating bath in order to improve the adhesion of zinc to the steel sheet. This Al reacts with Fe precipitated from the steel sheet to form Fe ₂ Al ₅ and floats on the bath surface as so-called top dross. Further, on the bath surface, there are foreign substances carried from the furnace by the steel plate and floating foreign substances due to zinc vapor in the snout. Moreover, because of the presence of snout, all these foreign matters are concentrated near the position where the steel plate enters the plating bath and adhere to the steel plate immediately before plating, or as the steel plate penetrates into the bath, the plating bath surface near the steel plate Is generated by the accompanying flow caused by the intrusion of the steel sheet, causing a surface flow from the snout wall surface to the steel sheet side, being inserted into the bath and adhering to the steel sheet traveling in the bath. As a result, plating defects such as indentation defects due to non-plating and dross adhesion occur. Even if these are not the major causes, the following new problems arise.

That is, especially the dross floating on the bath surface grows in particle size and grows with the passage of time, and the zinc-plated steel sheet adhered to the steel sheet is alloyed as it penetrates into the steel sheet bath. In this case, the dross directly adhered to the steel sheet is alloyed to prevent the dross from adhering to the galvanized layer when Fe in the steel sheet diffuses, resulting in poor appearance. That caused a new problem. In order to solve these problems, it is necessary to prevent the generation of dross. Therefore, by controlling the atmosphere on the plating bath in the snout, the generation of scum on the plating bath surface is prevented.

Although FIG. 1 shows the steps described above,
In such a process, nitrogen gas is supplied directly above the plating bath in the snout 2 from the atmosphere gas supply pipe 6 in which the oxygen concentration and the dew point are controlled, and the contact with oxygen on the bath surface in the snout is blocked, It prevents the generation of scum.
That is, in the snout, due to the presence of a small amount of oxygen and water contained in the reducing atmosphere gas, nitrides, oxides, hydroxides and the like are generated and float as scum on the plating bath surface in the snout. Therefore, in order to prevent scum floating on the bath surface in the snout, it is necessary to regulate the oxygen concentration to 10 ppm or less. The dew point is also -10 ° C.
By the following, it is possible to enhance the effect of preventing scum generation for the first time by controlling both the oxygen concentration and the control. However, in order to lower the dew point temperature to −40 ° C. or lower, it is necessary to install a large number of gas drying dryers, which causes a rise in equipment costs, and it is not possible to maintain and manage a large amount of atmospheric gas at a dew point −40 ° C. or lower. Difficult, therefore desirable -10 ℃
-40 degreeC.

FIG. 2 is a vertical sectional view showing a plating process which is another embodiment according to the present invention. 2 is basically the same apparatus as in FIG. 1, in which a steel plate 1 discharged from a continuous annealing furnace passes through a snout 2 and is immersed in a plating bath 3 in a plating tank, and then plated via a pot roll 4. In this case, the atmosphere gas supply pipe 6 in which the oxygen concentration and the dew point are controlled is arranged in the snout bath, and nitrogen gas is supplied from the atmosphere gas supply pipe 6 to form bubbles in the bath. This prevents the generation of scum and the adhesion of scum to the steel plate. Even in this case, the oxygen concentration is 10 ppm as the atmosphere gas.
Hereinafter, it is necessary to control the dew point temperature to be -10 ° C or lower.

[0014]

【Example】

Example 1 Using the device shown in FIG. 1, 0.8 mm (thickness) × 121
A 9 mm (width) steel plate is passed at a line speed of 80 mpm, and the oxygen concentration is -15 pp immediately above the Snout plating bath.
m, dew point temperature is -20 ° C, and nitrogen gas is adjusted to 100 Nm.
While blowing ³ / hr, Zn-0.1-0.3% Al,
The surface appearance after immersing in a hot dip galvanizing bath having a bath temperature of 460 ° C., plating and then wiping the hot dip galvanized steel sheet to a predetermined plating thickness after alloying treatment was observed. The result is shown in FIG. FIG. 3 shows the occurrence rates of surface appearance defects due to the adhesion of scum in the present invention and the comparative example. As shown in FIG. 3, by applying the method of the present invention as compared with the conventional method in which no atmospheric gas is blown in, the incidence of surface appearance defects due to scum adhesion could be significantly reduced.

Example 2 Using the apparatus shown in FIG. 2, 0.8 mm (thickness) × 121
A 9 mm (width) steel plate is passed at a line speed of 80 mpm, and the oxygen concentration is -20 pp just above the Snout plating bath.
m, dew point temperature of nitrogen gas adjusted to -20 ℃ 120Nm
While blowing ³ / hr, Zn-0.1-0.3% Al,
The surface appearance after immersing in a hot dip galvanizing bath having a bath temperature of 460 ° C., plating and then wiping the hot dip galvanized steel sheet to a predetermined plating thickness after alloying treatment was observed. As a result, the same effect as in Example 1 can be obtained, and by applying the method of the present invention as compared with the conventional method in which no atmospheric gas is blown in, the incidence of surface appearance defects due to scum adhesion is significantly reduced. Was completed.

[0016]

As described above, according to the present invention, it is possible to prevent the generation of scum on the bath surface, and even if a slight amount is generated, it is alloyed because the grain size is small, but the surface appearance is It has the excellent effect of being able to obtain a hot-dip galvanized steel sheet having an excellent surface appearance in which no abnormalities are observed.

[Brief description of drawings]

FIG. 1 is a vertical sectional view showing a plating process according to the present invention,

FIG. 2 is a vertical sectional view showing a plating process which is another embodiment of the present invention,

FIG. 3 is a diagram showing a surface appearance defect occurrence rate due to scum adhesion in the present invention and a comparative example.

[Explanation of symbols]

 1 Steel plate 2 Snout 3 Plating bath 4 Pot roll 5 Gas wiping nozzle 6 Atmosphere gas supply pipe

Claims (3)

[Claims] 1. A scum is prevented from occurring on the bath surface by supplying an atmosphere gas whose oxygen concentration and dew point are controlled directly above the plating bath in the snout where the steel plate is immersed in the plating bath at the snout portion of the molten metal plating. A hot-dip metal plating method characterized by the above. 2. A scum is prevented from occurring on the bath surface by supplying an atmosphere gas whose oxygen concentration and dew point are controlled into the plating bath in the snout in which the steel sheet is immersed in the plating bath at the snout portion of the molten metal plating. A hot-dip metal plating method characterized by the above. 3. A device for supplying an atmospheric gas, the oxygen concentration and the dew point of which are controlled, is arranged immediately above the plating bath in the snout in which the steel plate is immersed in the plating bath at the snout portion of the molten metal plating. And a molten metal plating device.